Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes

Problem 62

Chapter 11, Problem 62

Acetylide alkylation, from Assessment 12.61, fails to give the desired product with 2° haloalkanes. Why? What is the actual product of this reaction?

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Acetylide alkylation involves the reaction of a terminal alkyne with a strong base (e.g., NaNH₂) to generate an acetylide ion, which is a strong nucleophile. This nucleophile can then react with alkyl halides in an SN2 reaction to form a new carbon-carbon bond.

In the case of 2° (secondary) haloalkanes, the SN2 mechanism is hindered due to steric hindrance around the electrophilic carbon. The bulky groups attached to the secondary carbon make it difficult for the acetylide ion to approach and attack the carbon from the backside, which is a requirement for the SN2 mechanism.

Instead of proceeding via the SN2 pathway, the reaction with a 2° haloalkane is more likely to undergo an E2 (elimination) mechanism. The strong base (acetylide ion) abstracts a β-hydrogen from the 2° haloalkane, leading to the formation of an alkene as the major product.

The actual product of this reaction is therefore an alkene, not the desired alkylated alkyne. The specific alkene formed depends on the structure of the 2° haloalkane and the location of the β-hydrogens.

To successfully perform acetylide alkylation, it is recommended to use 1° (primary) haloalkanes, as they are less sterically hindered and favor the SN2 mechanism, leading to the desired product.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acetylide Nucleophiles

Acetylide ions are strong nucleophiles derived from terminal alkynes. They are capable of attacking electrophilic centers, such as carbon atoms bonded to halogens in haloalkanes. However, their reactivity is influenced by the structure of the haloalkane, particularly whether it is primary, secondary, or tertiary.

SN2 Mechanism

The SN2 mechanism is a type of nucleophilic substitution where the nucleophile attacks the electrophile from the opposite side of the leaving group, resulting in a concerted reaction. This mechanism is favored with primary haloalkanes due to less steric hindrance, while secondary haloalkanes often lead to competing elimination reactions instead of substitution.

Elimination Reactions

In the context of secondary haloalkanes, acetylide alkylation can lead to elimination reactions instead of the desired substitution. This occurs when the nucleophile attacks the haloalkane, resulting in the formation of alkenes through the loss of a leaving group and a hydrogen atom, particularly when steric hindrance prevents effective SN2 substitution.

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Related Practice

Textbook Question

Which of the following substitution reactions would you expect to occur more quickly? Explain your answer.

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Textbook Question

The introduction of elimination reactions provides a second way to synthesize alkynes in a two-step process starting with an alkene. Suggest a mechanism for both steps of this process.

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Textbook Question

Suggest a bromoalkane and the conditions necessary to produce the alkenes shown.

(a)

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Textbook Question

In Chapter 10, you learned how to make an alkyne by acetylide alkylation with a 1° haloalkane. Suggest a mechanism by which this reaction occurs.

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Textbook Question

When the reaction scheme in Assessment 12.63 is done on a monosubstituted alkene, at least three equivalents of base are needed. Reacting the product of step 2 with D–Cl (D is an isotope of H) incorporates deuterium at the terminal carbon. Explain these two observations.

Textbook Question

Suggest a bromoalkane and the conditions necessary to produce the alkenes shown.

(b)

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